The present invention relates to modems using QAM modulation techniques and in particular to a circuit for monitoring phase jitter.
Phase jitter and frequency offset are two line impairments which have a detrimental effect on the incoming signal in a digital modem receiver. To prevent these impairments from causing errors in the demodulated modem signal a phase tracking loop may be utilized, as disclosed in the commonly assigned co-pending application Ser. No. 407,451 filed on 8/12/82 which is incorporated herein by reference. Actually, the phase tracking loop is effective with respect to frequency offset and low frequency, low amplitude phase jitter while high amplitude phase jitter sometimes cannot be tracked out by the phase tracking loop.
In the past separate systems had to be provided for modems for measuring the phase impairment of the line. The present invention makes use of the phase lock loop error signal to derive and monitor the phase error of a line
In view of the above, it is the principal objective of the present invention to provide an improved circuit for a digital modem wherein the phase error function is derived from the phase lock loop circuit.
A further objective is to provide such a circuit which may be implemented in a conventional digital modem.
Still further objects and advantages will be apparent from the following description of the invention.